Hitherto, manganese dioxide has been mainly used as a positive electrode active substance in alkali batteries. However, since devices particularly requiring high output has been increased in accordance with remarkable developments of various portable electronic devices in recent years, developments of batteries more excellent in high rate discharge characteristics than those of most frequently used alkali manganese batteries have been expected.
Meanwhile, a sealed alkali zinc secondary battery comprising a positive electrode composed of nickel oxyhydroxide as a major construction material, a negative electrode using an alloy composed of zinc as major component, a separator and a metal container has been known (British Patent No. 365125). This battery is known to have excellent features such as (1) a battery voltage of as high as 1.73 V, (2) flatness of a discharge curve and (3) high utilization rate in high rate discharge. However, it was a problem that the electric capacity of this battery remarkably decreases during continuous or discontinuous discharge.
An inside-out nickel zinc secondary battery using nickel hydroxide as a positive electrode active substance and zinc as a negative electrode active substance has been also known (Japanese Patent Application Laid-Open (JP-A) No. 2000-67910). However, such battery involves a problem of leakage of electrolytes by generating oxygen gas from the positive electrode at the time of charging by repeating charge-discharge cycles to increase an inner pressure of the battery. Since this battery has a theoretical capacity ratio of 2:1 between the positive electrode and negative electrode, it is difficult to attain high capacitance.
It is also known that heavy load discharge characteristics after high temperature storage can be improved by using manganese dioxide and nickel oxyhydroxide as a mixture in the alkali battery (JP-A No. 2000-48827). In this invention, manganese dioxide and nickel oxyhydroxide are used as a cathode mix so that the blending ratios of manganese dioxide and nickel oxyhydroxide of 20 to 90 parts by weight and 80 to 10 parts by weight, respectively. Since a plateau characteristic of manganese dioxide appears at around 1.1 V when the cathode mix mainly composed of manganese dioxide is used as in the invention above, the high rate discharge capacity tends to vary and to lead to deterioration of the high rate discharge capacity at low temperatures to impair the use of the battery.
Based on the situation above, the applicant of the invention noticed it preferable that nickel oxyhydroxide is mainly used as a cathode mix in the devices requiring high rate discharge characteristics such as digital cameras, and have filed an application about a nickel zinc primary battery in which nickel oxyhydroxide is used as a positive electrode active substance (JP-A No. 2000-351812).
While the nickel zinc primary battery has been confirmed to be quite excellent in high rate discharge characteristics, the following problems remain to be improved in the battery using nickel oxyhydroxide only as the active substance of the cathode mix.
(1) When a hollow positive electrode is molded, a large quantity of nickel oxyhydroxide adheres on a mold to fail in obtaining the hollow positive electrode having sufficient moldability. This causes a decrease of discharge capacity during low rate discharge while it is disadvantage for designing a theoretical capacity ratio of the positive electrode to the negative electrode in an overdischarge process.
(2) While the theoretical capacity ratio of the negative to positive electrodes obtained by dividing the theoretical capacity of the negative electrode by the electrode theoretical of the positive capacity is preferably 1.0 or more considering the high rate discharge characteristics, the amount of hydrogen gas generated from the positive electrode increases because the amount of residual zinc during the overdischarge process increases as the theoretical capacity ratio of the negative to positive electrodes increases. On the other hand, while the ratio between the capacity of the negative electrode and positive electrode is designed by controlling the weight of the positive and negative electrodes in producing the battery, the maximum ratio is about 1.2 when the minimum ratio is set at about 1.0 since the tolerance of the ratio is about ±0.1. However, since the amount of hydrogen gas generated increases by overdischarge when the theoretical capacity ratio of the negative to the positive electrode exceeds 1.1, hermetic sealing of the battery cannot be maintained unless a safety valve actuating pressure is increased in designing the battery. While the safety valve actuation pressure of the primary battery is usually adjusted to about 5 to 8 MPa, an actuating pressure of the safety valve exceeding this level may cause problems on safety.